The emergence of glyphosate-resistant weeds has generated interest in the use of residual herbicides as tank-mix partners with glyphosate in glyphosate-tolerant (e.g., ROUNDUP READY or RR) crops. Acetamide herbicides, including, for example, acetanilide herbicides, typically do not offer significant post-emergence activity, but as a residual partner would provide control of newly emerging monocots and small-seeded dicot weed species. This would usefully supplement the activity of glyphosate which is effective on emerged weeds, but lacks significant residual activity.
Acetanilide herbicides have traditionally been applied to the soil before planting as pre-emergent herbicides. The application of acetanilide herbicides prior to emergence of the crop, however, has caused many crops to be damaged or killed. In response to this problem, it was proposed to apply commercially available acetanilide herbicide formulations after the emergence of the crop (i.e., post-emergent to the crop), but before the emergence of later germinating weeds (i.e., pre-emergent to the weeds). Application during this time window, however, caused unexpected foliar injury to the crop. The injury was observed with both commercially available conventional acetanilide emulsifiable concentrate (EC) formulations and with commercially available encapsulated acetanilide formulations.
Prior art microencapsulation procedures are generally adequate for producing formulations with good weed control. However, the practitioner of this art has had some difficulty optimizing the release rates to obtain acceptable bioefficacy for a given active while minimizing crop injury to commercially acceptable levels. In particular, commercial encapsulated formulations may show greater systemic crop plant injury over time in the form of leaf crinkling and plant stunting when compared to emulsifiable concentrates.
In microencapsulation technology known in the art, core herbicide is typically released from a microcapsule at least in part by molecular diffusion through the shell wall. Modification of shell wall thickness to increase or decrease herbicide rate has definite limitations.
Thin shell walls are sensitive to premature mechanical rupture during handling or in the field, resulting in immediate release. Poor package stability resulting from shell wall defects can also arise when the core material is in direct contact with the external vehicle. As a result, some core material may crystallize outside the capsule causing problems in spray applications, such as spray nozzle plugging. Further, higher shear encountered in certain application means, such as spray applications, can result in shell wall rupture and herbicide release. The microcapsule thus becomes little more than an emulsion stabilized against coalescence. When delivered to the field, herbicide release is so fast that little crop safety improvement is gained over conventional emulsion concentrate formulations.
If the wall thickness is increased, the bioefficacy quickly drops to a marginal performance level because herbicide release is delayed. There is also a practical limit to the wall thickness in interfacial polymerization. As the polymer precipitates, the reaction becomes diffusion controlled. The reaction rate can drop to such an extent that non-constructive side reactions can predominate.
Various formulation solutions have been attempted to address the release rate limitations. For example, two package or single package blends of microcapsules and dispersions or emulsions of free agricultural actives have been proposed in Scher, U.S. Pat. Nos. 5,223,477 and 5,049,182. Seitz et al., U.S. Pat. No. 5,925,595 and U.S. Publication No. 2004/0137031 A1, teach methods for producing microencapsulated acetochlor. The degree of permeability is regulated by a compositional change in the precursors for the wall. Although the Sietz compositions have proven effective for weed control, unacceptable crop injury has been observed in connection with the use of those compositions when applied to certain commercially important crops.
A need therefore exists for herbicide compositions and methods utilizing acetamide herbicides such as acetamide herbicides whereby simultaneous commercially acceptable weed control and commercially acceptable crop injury can be attained.